Gross cystic disease fluid protein-15 is one of the major components of cystic fluid and a well known marker of apocrine differentiation. Monoclonal antibodies raised against this protein are highly specific and sensitive as breast carcinoma markers. We will evaluate the usefulness of this monoclonal antibody to recognize breast cancers among metastasis and tumors of unknown origin; for this, a number of primary breast cancer tumors, other primaries, metastases and tumors of unknown origin will be studied in a blind fashion by immunoperoxidase in an attempt to identify those that represent breast cancers. These antibodies may be of tremendous assistance in female patients that present with carcinomas of unknown origin.